variance and standard deviation

[knightunner]

I've heard that the standard amount a good limit player makes is 1BB/ hour. What is the variance on this? I am wondering if I need to brush up on my understanding of the game since I have documented the (roughly) last 50 hrs of my playing and I am down 2.5 BB. Does this fall within the standard deviation of a good player? Or am I a losing player?

Also, could someone please explain the math behind this for me?

Thanks,
knight